Various methods are known for bonding separately formed portions of a piston in order to yield a piston structure. One such process is friction welding in which one portion of the piston is rotated at high speed while pressed against the other portion, with the resulting frictional energy generating sufficient heat to bond the portions together. Other techniques include resistance welding, induction welding, and the like in which, after the portions are brought into contact with one another, an energy flux is introduced across their joining surfaces which causes them to be heated sufficiently to join the surfaces to one another.
U.S. Pat. No. 5,150,517 is an example of friction welding, whereas U.S. Pat. No. 6,291,806 is an example of typical induction heating wherein the coils are presented to the sides of the contacting joining surfaces to induce energy and thus heat at the interface. Such side presentation of the induction coils has a tendency to heat the regions of the joining surfaces near the edges of the material adjacent the induction coils at a faster rate than those regions further from the coils, thus producing a variation in the heat flow and heat affected zone in the area of the material adjacent the interface. In a demanding, highly loaded application such as pistons for diesel engines, it would be desirable to provide a weld joint that is uniform in its heat affected zone across the interface so as to minimize any variation in strength and integrity of the material.
U.S. Pat. No. 6,155,157 discloses a piston having first and second portions which are joined across two radially spaced sets of joining surfaces by means of friction welding. It will be appreciated that such an architecture would present a challenge to joining the portions by induction welding, since access to the regions where the joining surfaces are located is limited and, in the case of the internal cooling gallery, inaccessible to the positioning of any induction coil next to the mated joining surfaces. Based on the known existing technology in the field of pistons, a suitable technique for induction welding such complex architectures of pistons as those shown in the aforementioned ""642 patent is not known to be in existence, and certainly is not known to be used due to the practical difficulties in adapting such induction heating technology to complex piston designs with multiple radially spaced joining surfaces.
Outside of the field of heavy-duty pistons, induction heating is used to join simple structures, such as butt-welding metal tubes that carry petroleum products. Such tubing is a simple, single walled cylindrical structure having flat, planer end faces. To join one end face to another, an induction coil is introduced between the end faces, and the end faces are heated to an elevated temperature, after which the coil is withdrawn and the end faces brought into engagement with one another to achieve a weld joint. Preferably, once the surfaces are brought into contact, they are twisted a small amount (a few degrees) to attain more intimate union of the weld surfaces. Surprisingly, the inventors have discovered that the induction welding technique heretofore limited to joining simple single walled cylindrical petroleum piping can be improved to be successfully employed to join complex piston structures in a manner to attain a strong, high integrity joint with a uniform but minimal heat affected zone across the interface of the joining surfaces.
A method of making a piston according to a first aspect of the invention includes fabricating first and second portions of the piston each having at least two joining surfaces. The portions are supported with the joining surfaces in spaced relation to one another. While spaced, the joining surfaces are heated to an elevated temperature and thereafter the heat discontinued and the joining surfaces brought into contact with one another to form a metallurgical bond across the joining surfaces.
According to another aspect in the invention, a method is provided for making a piston in which a joining surface of a first piston portion is supported in spaced relation to a joining surface of a second piston portion and, while spaced, the surfaces are heated and then brought together to form a metallurgical bond.
According to still a further aspect in the invention, a piston is provided having first and second portions with mating joining surfaces joined by an induction weld joint and having a heat affected zone which is uniform across the joint.
The invention has the advantage of providing a simple, low-cost method for welding multi-piece pistons.
The invention has the further advantage of providing a low-cost, high integrity weld joint that has a small and uniform heat affected zone adjacent the weld joint.
The invention has the further advantage of providing an induction heating method which permits precise control of the heating of the joining surfaces of the two piston parts, such that the joining surface of each piston part is not overheated or underheated during the heating of the joining surfaces to an elevated bonding temperature.
The invention has the further advantage of heating the joining surfaces of the piston portions, while spaced apart from one another, for a more precise, uniform and controlled heating of the surfaces as compared to heating the surfaces after they are joined to one another. With friction welding, for example, a piston having upper and lower crown parts with adjoining surfaces provided at the end faces of radially spaced inner and outer wall sections of the portions necessarily result in the outer wall being heated relatively more than the inner wall since the outer wall diameter is greater and thus rotates at a greater angular speed than that of the inner wall and consequently generates frictional heat at a greater rate than that of the heat generated at the inner wall. Unlike friction welding, induction heating makes it possible according to the invention to precisely control the relative heating of the inner and outer walls of such pistons, thereby providing more uniform weld joints as between the inner and outer walls.
Controlling the heating of inner and outer walls of the piston which are joined by the method of the invention avoids excessively heating the outer wall where the ring grooves are formed to better control the heat flow in the ring belt region as compared to friction welding.
Another advantage of induction heating according to the invention is that it requires relatively low compression force to join the parts following induction heating as compared to friction welding in which the heat needed for welding is generated by relative rotation of the parts while under relatively high compression loads (about 1,000 psi vs. 20,000 psi for friction welding). Consequently, the fixturing and equipment needed to hold and support the parts for induction welding according to the invention need not be as substantial as that required for friction welding. Moreover, the architecture of the piston is liberated somewhat since the structure does not have to withstand the heavy compression loading which is imparted during friction welding and which often exceeds the loading experienced during use of the piston. Consequently, thinner sections and lighter weight pistons are possible with induction welding at a cost savings to the manufacturer and recognized fuel and emission efficiencies by the user of such pistons.